An antenna is a transducer device that is operative for providing an interface between a wired circuit and a wireless propagation medium. As such, an antenna can be used as a transmit device to transmit signals to the wireless medium, as a receive device to receive signals from the wireless medium, or as a combination transmit/receive device to perform both of these functions. In many applications, an antenna will need to be appropriately characterized (e.g., calibrated, etc.) before transmit and/or receive operations are undertaken. Such characterizations may be performed before an antenna is deployed in the field and may also be performed periodically during the life of an antenna to compensate for error sources that change over time (e.g., circuit drift, etc.).
One type of antenna that is widely used is the array antenna. An array antenna typically includes a plurality of array elements that are located in fixed relation to one another. In some implementations, an array antenna may be made up of a plurality of identical or near identical radiating elements known as unit cells that are replicated throughout the array, although array antennas that include elements that are not technically unit cells also exist.
One popular technique for calibrating an array antenna involves the use of near-field measurements to deter nine calibration coefficients for the elements of the array. A point source probe antenna in the near-field of an array antenna under test (AUT) is sequentially positioned at each of the elements of the AUT. At each probe position, a test signal is transmitted to the corresponding antenna element of the AUT and a measurement is made at the element. A calibration coefficient is then generated for the array element based on the measurement. This process is repeated to generate calibration coefficients for all elements of the AUT. A problem that arises is that certain mechanisms within the near field region (e.g., scattering effects, multi-path, temporal instability, polarization isolation errors, etc.) can act as error sources that can compromise the accuracy of the measurements. Errors in the measurements can directly translate into errors in the calibration coefficients that are used to configure the antenna elements for use in the field
To overcome problems related to reflections and multi-path in the near-field region, time domain processing techniques have been used to attempt to de-correlate the multipath. However, such techniques are often significantly limited by the operating bandwidth of the test array, which limits the time resolution. Similar RADAR processing techniques also have limited effectiveness for short range scattering, which is the dominant error component.
Techniques and systems are needed for improving the accuracy of near-field array antenna calibration.